1. Field of the Invention
This invention relates to a method of damping nutational motion in satellites and other spacecraft systems, and more particularly to providing a smooth transition from a station-keeping mode in which the spacecraft is under thruster control to an on-orbit operational status in which control is maintained using momentum wheels to make small orientational corrections.
2. Description of Background Art
The improvements described in this disclosure incorporate by reference the subject matter described in U.S. Pat. No. 4,931,942 issued to Garg et al. on Jun. 5, 1990. The Garg patent describes a method for controlling nutational motion during spacecraft transition from a station-keeping mode to an on-orbit mode using a feedback control system to control multiple thruster pulse firings. Although the problems of thruster non-idealities and orbital dynamic nonlinearities were raised, no solutions were offered beyond convergence to stability through successive feedback controlled thruster pulses.
U.S. Pat. No. 4,288,051 issued to Goschel relates to the stabilizing of a satellite relative to the three major axes prior to the point in time when the satellite is to change orbits, whereupon the engine system for reaching the new orbit is switched on. No separate nutation-damping scheme is disclosed.
U.S. Pat. No. 4,537,375 issued to Chan describes a method of pre-biasing individual thruster motors to compensate for motor offsets and mismatches. The damping of nutational motion is not addressed.
U.S. Pat No. 4,725,024 issued to Vorlicek describes a method for spinning-up a three-axis controlled spacecraft. Nutational motion compensation is not described.
U.S. Pat. No. 4,758,957 issued to Hubert et al. discloses a method for simultaneously processing and nutation-damping a spinning spacecraft that includes thruster firing in response to feedback from angular momentum gyros. This patent has no disclosure of the subject three-pulse thruster firing scheme, nor does it address the topic of thruster compensation.
Other patents uncovered which contain additional information on the general topics of nutation, attenuation, correction in spacecraft systems and the like are as follows:
______________________________________ U.S. Pat. No. Inventor ______________________________________ 3,624,367 Hamilton, et al. 3,643,897 Johnson, Jr. 3,866,025 Cavanagh 3,937,423 Johansen 3,944,172 Becker 3,984,071 Fleming 3,997,137 Phillips 4,023,752 Pistiner, et al. 4,174,819 Bruederle, et al. 4,370,716 Armieux 4,386,750 Hoffman 4,521,855 Lehner, et al. ______________________________________